British Prime Minister David Cameron convened an Internet safety summit today. He summoned industry executives, watchdogs and law enforcement to Number 10 Downing Street. The result was an ambitious new initiative aimed at curtailing some, though not all, online access to child pornography. Vicki Barker has the story and the back-story.

VICKI BARKER, BYLINE: Last July, David Cameron issued a pointed challenge to the giants of the Web. The British prime minister, a father of young children, simply refused to accept the companies' argument that there was little they could do to stop people accessing indecent images of children.

PRIME MINISTER DAVID CAMERON: I have a very clear message for Google, Bing, Yahoo and the rest. You have a duty to act on this and it is a moral duty.

BARKER: Fail to act, Cameron suggested, and he'd push for legislation making it a legal duty. What followed was an intensive collaboration among Internet companies and watchdogs, law enforcement officials, and the British government. And today, Microsoft and Google, which, together, account for 95 percent of online searches, said they're introducing algorithms to block up to 100,000 search terms for child pornography. And other Internet firms are following suit. Peter Barron of Google.

PETER BARRON: Child sexual abuse is illegal in every country around the world. There's consensus on that. And it's absolutely right that we identify it, remove it, and report it to the authorities.

BARKER: People using Microsoft's Bing or Google or any other participating search engines to look for images of child sexual abuse will instead see a warning that it's illegal. Joanna Shields is the U.K.'s ambassador for digital industries. She was part of the consultation process and calls the breakthrough unprecedented.

JOANNA SHIELDS: In the tests, we have not been able to find any illegal material. And this is the first time that we've been able to say that, that the tests have proven that.

BARKER: The blocking software is being used for English-language terms at first. But it will be expanded to more than 150 languages and countries within six months. Conservative lawmaker Claire Perry.

CLAIRE PERRY: So people right across the globe will be safer as a result of the action that Britain's taking today.

BARKER: But Jim Gamble, a former police chief fighting child porn, says the new software will only catch the amateurs and dabblers.

JIM GAMBLE: Pedophiles do not go online and search out their images on Google or Bing or Yahoo, for that matter. The hardcore pedophile lives in the deepest, darkest recesses of the Internet.

BARKER: True, says Internet watchdog John Carr, but the most hardcore pedophiles started out as amateurs and dabblers. Carr says the new blocks could deliver a salutary shock to them.

JOHN CARR: For some of these guys, that will be enough to pull them up short. They'll stop and they'll never go back to it.

BARKER: Among the measures being piloted is software that blocks pathways to torrent networks, the almost-untraceable peer-to-peer networks favored by serious pedophiles. Speaking after his summit, the prime minister couldn't hide a certain note of triumph.

CAMERON: We really should be clear today that what we were previously told couldn't and shouldn't be done in terms of cleaning up searches for these vile terms, it will be done. It is being done. 100,000 terms in 159 countries, not just here in the U.K.

BARKER: Cameron says the next step should be another collaboration. This one, between British and U.S. intelligence officials, targeting the so-called dark Web, where the worst of the abuse takes place. For NPR News, I'm Vicki Barker in London. Transcript provided by NPR, Copyright NPR.